Lithographic printing is a printing process that makes use of the inherent immiscibility of water and oil. Lithographic printing plates used in lithographic printing have formed on a surface thereof regions which are receptive to water and repel oil-based inks (referred to below as “non-image areas”) and regions which repel water and are receptive to oil-based inks (referred to below as “image areas”).
The aluminum support employed in a lithographic printing plate (referred to below simply as a “lithographic printing plate support”) is used in such a way as to carry non-image areas on its surface. It must therefore have a number of conflicting properties, including, on the one hand, an excellent hydrophilicity and water retention and, on the other hand, an excellent adhesion to the image recording layer that is provided thereon. If the hydrophilicity of the support is too low, ink is likely to be attached to the non-image areas at the time of printing, causing a blanket cylinder to be scummed and thereby causing so-called scumming to be generated. In addition, if the water retention of the support is too low, clogging in the shadow area is generated unless the amount of fountain solution is increased at the time of printing. Thus, a so-called water allowance is narrowed.
Various studies have been made to obtain lithographic printing plate supports exhibiting good properties. For example, Patent Literature 1 discloses a method of manufacturing a lithographic printing plate support which includes a first step for anodizing a roughened aluminum plate surface and a second step for reanodizing under such conditions that the diameter of micropores may be smaller than that in the anodized film formed in the first step. It is described that the lithographic printing plate obtained using the lithographic printing plate support does not deteriorate the deinking ability in continued printing, improves the adhesion to the photosensitive layer, does not cause highlight areas to be blown out, and has a long press life.
On the other hand, printing may be suspended. In such a case, the lithographic printing plate is left to stand on the plate cylinder and its non-image areas may be scummed under the influence of the contamination in the atmosphere. Therefore, when the printing having been suspended is resumed, a number of sheets must be printed until normal printing can be made, thus causing wasted use of printing paper or other defect. It is known that these defects prominently occur in the lithographic printing plates having undergone electrochemical graining treatment in an acidic solution containing hydrochloric acid. In the following description, the number of sheets wasted when the printing having been suspended is resumed is used to evaluate the deinking ability after suspended printing and the deinking ability after suspended printing is rated “good” when the number of wasted sheets is small.
In addition, a large number of researches have been made on computer-to-plate (CTP) systems which are under remarkable progress in recent years. In particular, a presensitized plate which can be mounted for printing on a printing press without being developed after exposure to light has been required to solve the problem of wastewater treatment while further rationalizing the process.
One of the methods for eliminating a treatment step is a method called “on-press development” in which an exposed presensitized plate is mounted on a plate cylinder of a printing press and fountain solution and ink are supplied as the plate cylinder is rotated to thereby remove non-image areas of the presensitized plate. In other words, this is a system in which the exposed presensitized plate is mounted on the printing press without any further treatment so that development completes in the usual printing process. The presensitized plate suitable for use in such on-press development is required to have an image recording layer which is soluble in fountain solution or an ink solvent and to have a light-room handling property suitable to the development on a printing press placed in a light room. In the following description, the number of sheets of printed paper required to reach the state in which no ink is transferred to non-image areas after the completion of the on-press development of the unexposed areas is used to evaluate the on-press developability, which is rated “good” when the number of wasted sheets is small.